324 EMBRYOLOGY. 



is attached to the common pedicle by a particular branch, the 

 future bronchi ; the pedicle has farther extended, as the trunk 

 of the trachea. 



In the course of the first half of the third day, a small 

 vesicular-looking protuberance arises from th rt intestinum 

 rectum (fig. 339, n) ; this proves to be the allantois, which 

 grows into the caudal involucrum, and distends it. The al- 

 lantois is covered externally with a stratum of the vascular 

 layer (fig. 343, B, e, d), which it carries with it in its growth. 

 The growth of this part is very rapid, in the course of the 

 fourth day (figs. 341, 345, n) forcing its way through the 

 caudal involucre, and the part by which it is attached being 

 drawn out into a hollow pedicle. The external covering from 

 the vascular layer shows ramifications of the aorta, which 

 form a beautiful vascular rete. On the fifth day, the allantois 

 presents itself as a large peduncuhted bladder protruding 

 from the umbilicus (fig. 344, 5), which, bending to the right, 

 has penetrated between the mesenteric and ventral lamina, and 

 lies betwixt the amnion .and the serous envelope. At this 

 time, the allantois is nearly as large as the entire embryo 

 (fig. 344), being almost five lines in diameter,* 



THIRD PERIOD IN THE HISTORY OF THE DEVELOPMENT OF THE 

 INCUBATED EGG : FROM THE COMMENCEMENT OF THE CIR- 

 CULATION IN THE ALLANTOIS TO THE EXCLUSION OF THE 

 EMBRYO. 



[ 493. The third and last period comprises the interval 

 from the sixth to the twenty-first day. The two first days, 

 however, comprehend almost all of general physiological inte- 

 rest which happens in this period, so that a shorter review 

 of the grand features of the changes which take place in the 

 embryo and ovum through its course will be sufficient. If 

 the egg be opened at the beginning of this period, it must be 

 done with great care, as the albumen has now entirely disap- 

 peared, and the embryo lies close to the membrane of the 

 shell ; the vitellary membrane has become exceedingly thin, is 

 very easily torn, and indeed is soon resolved entirely ; the 

 air-space at the blunt end of the egg has greatly increased in 



* According to Rathke, the lungs are evolved from the first as a pair ; 

 he describes them, on the fourth day of the incubation, as two small, 

 laterally compressed, thin laminae, tapering off from before backwards, and 

 ending in a blunt point, which spring from the oesophagus. 



